Monday, 5 November 2012

Ever seen anything like this? Eh, no...

That's the (inofficial) story behind the name of our recently described new rhynchocephalian Oenosaurus muehlheimensis. When the remains of this animal, which only consist of a crushed skull that was exposed in palatal view, and both mandibles, were found at the Schaudiberg quarry at Mühlheim, the quarry owners and the scientists working with them were first at a loss as to what animal this was. Anybody whom they showed it to had ever seen anything like that before, and my first guess, when shown photos of the skull in palatal view, was that it might be a chimeran chondrichthyan. Later we wondered whether it might represent a late surviving rhynchosaur, with the typical maxillary tooth battery that these animals possess. Only when I examined the mandibles, finally recognition dawned: these elements, with their high coronoid processes, enlarged mandibular foramen and convex articular surface, clearly indicated that we were dealing with a sphenodontian (=rhynchocephalian).

Preparation and careful examination and comparison of the skull confirmed this identification. Oenosaurus really represented a rhynchocephalian, though with a dentition as it has never been described before in a tetrapod. The dentition consists of massive tooth plates, which, under closer inspection, seem to be made up of hundreds or thousands of fused individual teeth, with small internal cavities and concentric arrangements of dentine layers around them. That's what it looked like when examining the tooth plates under the microscope, and our first interpretation was that these plates indeed represented simply fused individual teeth, possibly including several tooth generations, as in the tooth batteries of some ornithischian dinosaurs. Given that rhynchocephalians usually do not show tooth replacement, this would have been weird enough, but then we made a computer tomography of the tooth plates. The results showed no evidence of replacement teeth, but apparently continously growing dentine tubules that were fused into a single structure towards the surface and sometimes even showed branching patterns. A literture survey revealed that similar structures are basically only found in chimeran chondrichthyans and lungfishes, where this tooth tissue is called osteodentine or petrodentine.

Right mandible of Oenosaurus in lateral view.

Rhynchocephalians are an ancient lineage of lepidosaurian reptiles, the group that modern lizards and snakes also belong to. However, in contrast to the latter, which are currently represented by several thousand species, only two species of rhynchocephalians survived to the present day, both in the genus Sphenodon. This genus, commonly named the Tuatara, is currently restricted to s few islands off the coast of New Zealand, where these animals have found their last refuge. Since Sphenodon belongs to such an ancient lineage and also shows some rather primitive looking features, it is often considered a living fossil, and was consequently used frequently in studies relating to allegedly ancestral conditions for modern lizards, also in recent times.

Photo of the rather sympathetic looking Sphenodon, the only recent rhynchocephalian (courtesy Helmut Tischlinger).

However, palaeontological research in recent decades had already shown that many of the alledgedly primitive characters of Sphenodon are actually secondarily derived, and that rhynchocephalians were a diverse and successfull branch of the lepidosaurian tree at least in the early to mid-Mesozoic. Nevertheless, rhynchocephalians kept their status as "evolutionary loosers": since they seemed to have been inferior to lizards in the adaptability, they were doomed to dwindle and almost vanish. Only recently, research by Hugo Reynoso, Sebastian Apesteguía and Marc Jones, among others, has forcefully shown that rhynchocephalians were not only systematically, but also ecologically diverse and highly successful. With its extremely modified tooth plates, indicating a crushing dentition, an adaptation previously unrecorded in rhynchocephalians, Oenosaurus underlines this high evolutionary plasticity of the group and thus seriously calls into question the idea of their inferiority. This is another nice example that judging groups of animals by their recent representatives alone might greatly underestimate their true nature and potential, and that the very concept of a "living fossil" might be seriously flawed. The whole story can, though admittedly always still incomplete, only be told by incorporating the fossil record.

7 comments:

How does the dental development of Oenosaurus compare with the growth of the dentine tubules in desmostylian afrotherian mammals? I know they're totally different in body size, phylogenetic position, and geologic age, but they too have these tubules that apparently keep growing and fuse to adjacent tubes.

Hi!Sorry for taking so long with the answer! You made a good point. We thought about desmostylian mammals, but the teeth of Oenosaurus are different. As far as I understand desmostylians have teeth with closely spaced, high cusps, which consist of a pulp cavity, surrounded by dentine and enamel, and this happens to each of the teeth on each jaw. Oenosaurus has a single large tooth on each jaw. Each tooth plate consists of a multitude of dentine tubules, but all of them belong to a single large tooth. You recognize that on the oclusal view, because there is no enamel around the dentine tubes.Cheers!Adriana

I was very excited when I read about this in the news, but the etymology of the genus's name irritated me. I am originally from the Franconian Alb, but it has never occured to me that it might be a famous wine area. Does this only apply to the region around Mühlheim? Have I missed the grand cru of western bavaria? :)

Leaving that aside: great work, and again I'm thankful for you to publish in Plos One.

Hello! I simply wanted to highlight the fact that you actually succeeded in organizing a splendid site. Also I want to know one thing. Do you take into considerations writing as a professional or having a blog is basically just a kind of hobby of yours?

@Perisoreus:The area of Mühlheim is certainly not well known for wine, but the Franconian wine in general is famous. In any way, we needed some official etymology for the new taxon, apart from the story of the title of this post... ;-)